In the TOFMS, a preset amount of kinetic energy is imparted to ions originating from a sample component to make them fly a preset distance in a space. The period of time required for this flight is measured, and the mass-to-charge ratios of the ions are determined from their respective times of flight. Therefore, when the ions are accelerated and begin to fly, if the ions vary in the position and/or the amount of initial energy, a variation arises in the time of flight of the ions having the same mass-to-charge ratio, which leads to a deterioration in the mass-resolving power or mass accuracy. One commonly known solution to this problem is an orthogonal acceleration TOFMS (which is also called a perpendicular acceleration or orthogonal extraction TOFMS), in which ions are accelerated and sent into the flight space in a direction orthogonal to the incident direction of the ion beam (for example, see Non-Patent Document 1 or 3).
FIG. 11(a) is a schematic configuration diagram of a typical orthogonal acceleration TOFMS, and FIG. 11(b) is a potential distribution diagram along the central axis of the ion flight. Ions which have been generated in an ion source (not shown) are given an initial velocity in the X-axis direction and introduced into an orthogonal accelerator section 1. In this section, a pulsed electric field is applied between a push-out electrode 11 and each of the grid-like electrodes 12 and 13, whereby the ions are ejected in the Z-axis direction and begin to fly in a field-free flight space 2A inside a TOF mass separator 2. In the reflecting region 2B, where a rising potential gradient is formed, the ions are made to reverse their direction and travel backward, to eventually arrive at and be detected by a detector 3.
To suppress a deterioration in the mass-resolving power due to a spatial spread of the ions in the orthogonal accelerator section 1, the system is typically tuned so that an ion packet (a collection of ions) ejected from the orthogonal accelerator section 1 is transiently focused on a focusing plane 21 located in the field-free flight space 2A, and subsequently, the dispersed ion packet is once more focused on the detection surface of the detector 3 by the reflecting region 2B. To achieve such a focusing, the orthogonal accelerator section 1 may be either a dual-stage type in which two uniform electric fields are created with two grid-like electrodes 12 and 13 (as shown in FIG. 11(a)) or a single-stage type in which a single uniform electric field is created with one grid-like electrode. Similarly, the reflecting electric field created with the grid-like electrodes 22 and 23 may also be a dual-stage type with two uniform electric fields or a single-stage type with one uniform electric field. In any of these cases, what is necessary is to adjust the strengths of a plurality of uniform electric fields so as to make the ion packet focused on the detection surface of the detector 3. A theory for realizing such a focusing condition is described in detail in Non-Patent Document 1.
As described previously, in the orthogonal acceleration TOFMS, a grid-like electrode made of a conductive material is widely used to create the orthogonal acceleration electric field or the reflecting electric field. The “grid-like” structures in the present description include both a structure in which thin members are meshed in both horizontal and vertical directions in a grid-like (cross-ruled) pattern and a structure in which thin members are arranged at regular intervals (which are typically, but not necessarily, parallel to each other). An electrode having the former structure is often simply called a grid electrode, while an electrode having the latter structure may be called a parallel-grid electrode for the sake of distinction from the former type.
FIG. 12 is a partially-sectioned perspective view of one example of the conventionally used grid-like electrodes. This grid-like electrode 200 has a structure with crosspieces 201 of width W and thickness T aligned in parallel at intervals P. The opening 202 between the two neighboring crosspieces 201 has a width (smaller dimension) of P-W and a length (larger dimension) of L. The depth of the opening 202 is equal to the thickness T of the crosspieces 201.
In the case where there is a difference in the electric-field strength between the entrance side and the exit side (upper and lower sides in FIG. 12) of the grid-like electrode 200, if the width P-W of the opening 202 is excessively large, a noticeable dispersion of the beam occurs due to the penetration of the electric field through the opening 202 or the lens effect. Therefore, the width P-W of the opening 202 should be as small as possible. On the other hand, the transmission efficiency of the ions through the grid-like electrode 200 having the previously described structure is geometrically given by the ratio of the width of the opening 202 to the interval of the crosspieces 201, i.e. (P-W)/P. Accordingly, given the same interval P of the crosspieces 201, the ion transmission efficiency increases with a decrease in the width W of the crosspiece 201. To realize an ideal grid-like electrode which can achieve a high ion transmission efficiency and with low dispersion of the ion beam, the interval P and the width W of the crosspieces 201 should preferably be as small as possible. However, as will be explained later, those sizes have lower limits associated with the mechanical strength or manufacture feasibility.
Fine-grid electrodes for TOFMS manufactured using the technique of electroforming have been developed to achieve a high ion transmission efficiency while minimizing the interval P of the crosspieces 201. For example, Non-Patent Documents 2 and 3 disclose a grid-like nickel (Ni) electrode produced by electroforming, which measures 83 μm in the interval P of the crosspieces, approximately 25 μm in the width W of the crosspieces, and approximately 10 μm in the thickness T of the crosspieces. According to those documents, its ion transmission efficiency is approximately 70 to 80%. An example of commercially available grid-like electrodes is a product disclosed in Non-Patent Document 4. This product, which consists of tungsten wires with a diameter of 18 μm tensioned at intervals of 250 μm, has achieved a high ion transmission efficiency of 92%.
However, the conventional fine-grid electrodes which have been realized by electroforming, thin-wire tensioning or other techniques in the previously described manner are comparatively low in mechanical strength and hence have a problem as follows:
A dispersion in the initial kinetic energy of the ions in the Z-axis direction within the orthogonal accelerator section 1 causes a decrease in the mass-resolving power of the TOFMS. A turnaround time TA [i.e. the time-of-flight difference between two ions having the same initial position and the same initial kinetic energy, one ion moving in the same direction as the ion-extracting direction (i.e. in the positive direction of the Z-axis) and the other ion in the opposite direction (i.e. in the negative direction of the Z-axis)], is calculated by the following equation (1):TA∝√{square root over ((mE))}/F   (1)where F is the strength of the ion-extracting electric field in the orthogonal accelerator section 1, E is the initial kinetic energy of each ion, and m is the mass of each ion. This equation (1) suggests that strengthening the electric field in the orthogonal accelerator section 1 is effective for reducing the turnaround time TA. As one example, FIG. 13 shows the result of a calculation of the relationship between the extracting electric field and the turnaround time TA for an ion of m/z 1000 in a thermal motion (E=30 meV). For example, the result shows that, if the turnaround time TA must be reduced to 1 [ns] (1.0E-09s) or less to achieve a high mass-resolving power in the TOFMS, an electric field stronger than 1500 [V/mm] is required.
Strengthening the electric field in the orthogonal accelerator section in this manner increases the difference in the electric-field strength between the ion entrance side and the exit side of the grid-like electrode and thereby causes a strong force to act on the crosspieces of the grid-like structure. This force acting on the crosspieces increases as the electric field is made stronger to further reduce the turnaround time. For example, a calculation shows that the force acting on the grid-like electrode per unit area under an electric-field strength of 1500 [V/mm] is as high as 10 [N/m2]. According to a study by the present inventor, Currently known grid-like electrodes having the previously described structures can hardly bear such a force. For example, if a grid-like electrode made of nickel (Young's modulus=200 GPa) measuring W=20 μm, T=10 μm and L=30 mm and having an ion transmission efficiency of 80% is tested as a both-ends-fixed beam with a uniformly distributed load, the displacement in its central portion is estimated at approximately 6 mm, in which situation the crosspieces in the grid-like structure will probably be easily broken. FIG. 14 shows the result of a calculation of the predicted amount of displacement in the central portion of the crosspiece for various thicknesses T of the crosspiece under the previously described conditions.
In the case of a structure in which thin wires are used as the crosspieces, the previously described breakage can be prevented by using thicker wires. However, the use of thicker wires increases the width W of the crosspieces and sacrifices the ion transmission efficiency. A possible idea for increasing the mechanical strength using thin wires instead of thick wires is to decrease the length L of the openings. However, this design also sacrifices the ion transmission efficiency. In the case of manufacturing the fine-grid electrode using electroforming, the thickness T of the electrode should not be substantially increased, since the manufacturing process includes the step of peeling off a thin metal plate from a mold. Therefore, it is difficult to increase the mechanical strength while maintaining the small width W of the crosspieces. Stacking a plurality of electroformed grid-like electrodes one on top of another with high positional accuracy and bonding them together to increase the mechanical strength might also be possible. However, this idea is impractical from technical points of view as well as in regards to the production cost.
Furthermore, if the difference in the electric-field strength between the ion entrance side and the ion exit side of the grid-like electrode is large, the electric field penetrates through the openings of the grid-like electrode and adversely affects the mass spectra even if the openings have a small width. For example, in the system shown in FIG. 11(a), when ions are to be introduced into the space between the push-out electrode 11 and the first grid-like electrode 12, both the push-out electrode 11 and the first grid-like electrode 12 are set at the ground potential, while the second grid-like electrode 13 is set at a higher potential for extraction and acceleration. In an ideal situation, the introduced ions undergo no force in the Z-axis direction and travel straight in the X-axis direction. When the introduced ions are to be ejected, a pulsed voltage is applied to both the push-out electrode 11 and the first grid-like electrode 12 to create an electric field, by which the ions are ejected into the TOF mass separator 2. However, the extracting and accelerating electric field created by the second grid-like electrode 13 actually leaks through the openings of the first grid-like electrode 12 into the orthogonal accelerator section 1 in the ion-introducing process. This electric field has the effect of accelerating the ions in the Z-axis direction and curving their trajectories before ejection, which results in a deterioration in the mass-resolving power. The leaking electric field also makes the introduced ions continuously flow into the field-free flight space 2A within the TOF mass separator 2 before ejection, causing an increase in the background signal in the mass spectrum.
To address this problem, a system disclosed in Patent Document 1 has an increased number of grid-like electrodes in the orthogonal accelerator section 1 to create a potential barrier which prevents ions from leaking into the field-free flight space 2A after the ions have been introduced in the space between the push-out electrode 11 and the grid-like electrode 12. In a system described in Patent Document 2, which does not use a grid-like electrode in the orthogonal accelerator section 1, a potential barrier similar to the one described in Patent Document 1 is created by switching a voltage applied to an aperture electrode placed between the ion-accelerating region and the field-free flight space, so as to prevent the leakage of ions from the ion-accelerating region into the field-free space. In the technique described in Patent Document 1, the increase in the number of grid-like electrodes leads to an increase in the production cost as well as a decrease in the ion transmission efficiency. The technique described in Patent Document 2 also makes the production cost higher since it requires an additional element for switching the voltage.